“Flash memory”, i.e., electrically erasable programmable read only memory (EEPROM) devices are electrically erasable, non-volatile memory devices fabricated with tunnel oxides and high voltage transistors for programming and erasing the devices.
Most flash EEPROM cells have a double polysilicon structure with the upper polysilicon layer patterned to form the control gates and the word lines of the structure. The lower polysilicon layer is patterned to form the floating gates. The lower polysilicon layer is deposited on a tunnel oxide, which is thermally grown on a silicon substrate. A dielectric layer separates the two polysilicon layers. The interpoly dielectric can be an oxide-nitride-oxide (ONO) structure. The nitride layer of the ONO structure is a memory nitride with the ability to store a charge and prevent electrons in the floating gate from escaping.
The dielectric ONO structure is formed by depositing a first layer of silicon dioxide. A layer of silicon nitride is then deposited on top of the silicon dioxide. Finally, the second layer of silicon dioxide is formed on the nitride layer. The second oxide layer is typically grown in a thermal oxidation process, such as a steam oxidation, of the nitride layer.
Several problems exist with the way in which the ONO structure is currently formed. First, thermal oxidation of the nitride layer is a slow process. As the second oxide grows, oxygen in the chamber must migrate further through the increasingly thicker second oxide layer to reach the silicon in the nitride layer. In addition, thermal oxidation decreases the thickness of the nitride layer, because the second oxide layer oxidizes silicon atoms in the nitride layer. As a result, the final thickness of the ONO structure can be difficult to determine. With the scaling down of the physical dimensions of the memory cells for next generation high density non-volatile memory devices, it is particularly important to be able to determine accurately the final thickness of the interpolysilicon ONO structure.
One approach is to deposit the second oxide on the nitride layer. Depositing oxide onto the nitride layer is faster than the oxidation process and does not significantly alter the thickness of the nitride layer. However, deposition of the oxide layer provides a second oxide layer that is more susceptible to leakage current. A id deposited oxide has inferior material properties compared to a thermally grown oxide, affecting the overall integrity of the ONO structure.
Accordingly, there is a need for an improved method of forming an ONO structure. The method should provide a faster process of forming the ONO structure. In addition, the method should improve the integrity of the ONO structure.